This application claims the priority of German patents 198 37 507.7 and 198 37 509.3, both filed Aug. 19, 1998, the disclosures of which are expressly incorporated by reference herein.
The invention relates to an exhaust system of an internal-combustion engine having an exhaust gas purification arrangement as well as a storage container which can be evacuated and into which at least a portion of the internal-combustion engine exhaust gas flow can be introduced for a defined time period, particularly after a start of the internal-combustion engine. Concerning the known prior art, reference is made, in addition to German Patent Document DE 43 42 296 C1 and German Patent Document DE 40 25 565 A1, particularly to British Patent Document GB 1 349 051.
As known, the exhaust gases of a motor vehicle internal-combustion engine must be purified, that is, must be freed at least partially of harmful constituents, for which particularly exhaust gas catalysts are used. It is also known that these exhaust gas catalysts require a certain operating temperature so that they can fulfill their function of converting harmful exhaust gas constituents. Modern exhaust gas catalysts reach this so-called light-off temperature immediately after a (cold) start of the internal-combustion engine in conventional exhaust gas testing cycles only after approximately 25 seconds so that, during this time period--which in the following is also called a "critical" time period--, the internal-combustion engine emissions arrive in the environment in a virtually unpurified state.
As a remedial measure for these problems, it has been suggested to convey the internal-combustion engine emissions during this above-mentioned (critical) time period of, for example, 25 seconds into a storage container and to store it there until the exhaust gas catalyst has reached its light-off temperature or generally until the exhaust gas purification arrangement has reached is operationally ready state. Subsequently, the exhaust gas quantity situated in the storage container can then be fed to the operationally ready exhaust gas purification arrangement for the purification and/or to the internal-combustion engine (or its combustion space) for another combustion.
In practice, for storing the exhaust gas quantity emitted during the above-mentioned "critical" time period by the internal-combustion engine even at a low load (for example, during idling or the like), in addition to a relatively large storage container, a high-capacity feed pump is required which is abruptly completely active during a start of the internal-combustion engine and by way of which the exhaust gas quantity occurring in this time period is then delivered into the storage container in a pressurized manner.
The above mentioned British Patent Document GB 1,349,051 contains the information that the once filled storage container can be optimally evacuated, whereby the storage capacity for the next cold start is to be increased simultaneously. However, the latter aspect does not seem very plausible.
It is an object of the invention to indicate improvements of this known prior art. The solution of this object is characterized in that, in addition to a stop valve which maintains the vacuum in the storage container when the internal-combustion engine is stopped, a vacuum pump is provided for generating the storage vacuum. By way of such a vacuum pump, a vacuum can generally be generated in the storage container and, by means of the stop valve, can be maintained such that, during the above-mentioned critical time period, by means of this vacuum, the internal-combustion engine emissions are delivered securely and efficiently into the storage container. In particular, also in the event of a possibly required repetitive start of the internal-combustion engine or after relatively long stoppage times of the internal-combustion engine, by means of such a vacuum pump, the vacuum desired in the storage container can be generated in a reliable manner, even before the internal-combustion engine is started.
Thus, in a branch pipe, which leads the internal-combustion engine emissions flow to the storage container, a control valve may be provided for the apportioned charging of the storage container, whereby, subsequent to a start of the internal-combustion engine, during the charging of the storage container which therefore takes place in a desired manner, the vacuum which first exists therein is not reduced abruptly but virtually in an apportioned manner. This permits a removal by suction of internal-combustion engine emissions from the internal-combustion engine exhaust pipe over a longer period of time. For the best-possible implementation of this method, in addition to a pressure sensor detecting the pressure value in the storage container, a control unit is preferably provided which appropriately controls the vacuum pump and/or the control valve by means of the pressure sensor signals.
In addition, a suitable adsorber material for an undesirable exhaust gas constituent may be provided in the storage container, so that the exhaust gases charged into the storage container are simultaneously at least partially purified before, after the expiration of the desired or required storage time period, they are fed back into the exhaust pipe of the internal-combustion engine.
In a further, particularly preferred embodiment, the exhaust system can be constructed to be double-walled in areas so that two, essentially mutually coaxial exhaust gas pipes exist, in which case, by way of the first, preferably interior exhaust gas pipe, the exhaust gas is guided away from the internal-combustion engine and is guided by way of the second, preferably outer exhaust gas pipe to the storage container.
By means of an exhaust system which, in areas, has a double-walled construction shown, for example, in German Patent Document DE-AS 22 22 498, the internal-combustion engine emissions are therefore first guided away from the internal-combustion engine in a first exhaust pipe and are then guided to the storage container, specifically back in the direction of the internal-combustion engine, in a second exhaust pipe which extends coaxially and concentrically to the first exhaust pipe. This double-walled section of the exhaust system is therefore similar to a counterflow heat exchanger. As the result of this measure, the exhaust system is heated more intensively in the double-walled section, which promotes a faster light-off of the exhaust gas catalyst or catalysts; that is, as a result of this measure, the exhaust gas purification system reaches its operating temperature sooner.
This increased heating is accompanied by an increased cooling of the exhaust gas flow guided to the storage container, whereby the volume of the exhaust gas quantity or the exhaust gas mass to be actually stored is reduced according to the physical state equation for gases. A larger exhaust gas quantity or exhaust gas mass can therefore be stored in the spatially limited storage container. In this case, it is particularly advantageous for the transition between the first and the second exhaust gas pipe to be provided with respect to the internal-combustion engine downstream of the exhaust gas purification arrangement, while an outlet from the second exhaust pipe into a branch pipe leading to the storage container is situated between the internal-combustion engine and the exhaust gas purification arrangement. The reason is that the exhaust system then has a double-wall construction also in the area of the exhaust gas purification arrangement, whereby the latter experiences a particularly intensive heating by internal-combustion engine emissions.
Another advantage of an exhaust system according to the invention is the fact that the exhaust system, which has a double-wall construction in areas, itself forms a portion of the storage container, specifically with a minimal additional space requirement, so that the actual storage container can be designed to be correspondingly smaller. In addition, it is particularly recommended to feed the exhaust gas quantity first stored during the so-called "critical" time period after the operational readiness has been reached to the exhaust gas purification arrangement of the internal combustion engine for (another) afterburning so that the storage container should preferably be arranged close to the internal-combustion engine. By means of an exhaust system according to the invention, a relatively simple exhaust gas guiding to the storage container arranged close to the internal-combustion engine is thus advantageously permitted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.